1. Field of the Invention
This invention relates to a flat float glass that can be prestressed or transformed into a glass ceramic with high quartz mixed crystals or keatite mixed crystals.
2. Background of the Invention
Numerous applications of glass require flat glass, e.g. glass in the form of glass sheets or panels to be used as view windows and for display purposes. These flat glass items are manufactured from molten glass using methods of the prior art such as rolling, drawing, casting and floating.
On account of high surface quality requirements, special importance has been acquired by float glass, which is widely used in the glass industry. In the manufacture of glass sheets using the float glass method, a ribbon of glass is drawn from the glass-melting furnace and then, to smooth the surface, the glass, while still deformable, is drawn over a metal bath (e.g. consisting of molten tin). The glass thereby “floats” on the liquid metal. After it is removed from the metal bath, the glass sheet is cooled and then cut to length into individual segments. The result is a flat glass that has parallel and fire-polished surfaces and a high surface quality.
Using the float glass method, flat glass can be manufactured from different glass materials. For example, the prior art describes the use of the float process to manufacture mirror glass, such as borosilicate glass or aluminosilicate glass, for example, in addition to conventional soda lime glass.
Soda lime glasses are defined as glass that is manufactured essentially from the raw materials silicon dioxide (SiO2), lime (CaCO3) and soda (Na2CO3). Soda lime float glass can thereby also be thermally prestressed and thus meets the requirements for safety glass. To achieve this thermal prestress, the prior art describes the heating of cut-to-length sheets of float glass to approximately 600° to 700° C., followed by a very rapid cooling by a current of cold air. As a result of this heating and rapid cooling process, strong compression stresses are induced in the surface of the glass, along with tensile stresses in the interior of the glass. This combination of stresses results in a significant increase in the bending tensile strength of the glass, a lack of sensitivity to rapid temperature fluctuations and high elasticity. When extremely severe loads are applied, this type of prestressed glass breaks into a plurality of pellet-like pieces that do not have very jagged edges. Soda lime float glass is therefore used widely, e.g. in architectural glazing or as curved windows for glazing automobiles.
Soda lime glass can also be chemically prestressed.
In chemical prestressing, the compression stress in the surface of the glass is achieved by ion exchange. Larger-radius ions that penetrate into the glass from outside thereby replace smaller ions. As a result of the greater amount of space occupied by the penetrating ions, a high compression stress is achieved in the surface, which increases the strength of the glass by a factor of 5 to 8.
The ion exchange is generally performed using alkali atoms, either in fused salts or by means of applied pastes. One conventional treatment uses potassium atoms that replace sodium atoms in the glass. One essential requirement is that the treatment take place below the transformation temperature of the glass, because otherwise the compression stress decreases significantly because of the heat. Chemically hardened glass of this type is used for special applications, such as in the aviation and aerospace industries, for centrifuge glass and in the lighting sector.
Borosilicate glass is a silicate glass that contains between 7% and 15% boron oxide. On account of its composition, it has a high temperature resistance and a very high hydrolytic and acid resistance. Like soda lime glass, it can be floated and thermally prestressed and is fined during manufacture using NaCl. Borosilicate float glass is therefore used in applications that require increased chemical resistance and increased temperature stability, or heat resistance.
Aluminosilicate glass is a silicate glass that has as its essential component aluminum trioxide as well as other oxides. This category includes glass from the LiO2—Al2O3—SiO2 system. A glass of this type can also be floated and thermally prestressed, and can be fined with SnO2, for example. Aluminosilicate float glass also has an improved chemical resistance and higher temperature stability, and also has the advantage that it is free of alkali components, which makes it suitable for use as a substrate glass in display technologies.
It is generally known that aluminosilicate glass from the LiO2—Al2O3—SiO2 system can be converted into glass ceramics with high quartz mixed crystals or keatite mixed crystals as the principal crystal phases. Glass ceramic therefore consists of a crystal phase and a residual glass phase. The initial glass is obtained by normal glass manufacturing methods. After melting and fining, the glass conventionally undergoes a hot forming by rolling, casting or pressing. Then the glass is subjected to a heat treatment, as a result of which the glass is partly transformed under controlled conditions into a fine-grain crystal phase.
A key characteristic of these glass ceramics is that they can be used to manufacture materials that, in the temperature region of room temperature to about 700° C., have an extremely low coefficient of thermal expansion α20/700<1.5×10−6/K. These glass ceramics are therefore used in transparent form, for example for fireproof glass, as view windows in stoves and furnaces or as cookware, as well as for substrate material for wafer stages or mirrors for telescopes. The transparent glass ceramics can be colored dark by the addition of colored oxides. This dark coloration is desirable, for example, when the glass is used as a cooking surface, to conceal the technical components that are installed underneath the cooking surface.
In the large-scale industrial manufacture of glass ceramics, arsenic oxide and/or antimony oxide are generally used as fining agents. These fining agents are compatible with the required glass ceramic characteristics and result in good seed, or bubble, qualities or low numbers of seeds, or bubbles, in the melt. Even if these substances are incorporated into the structure of the glass, they have disadvantages from the point of view of safety and environmental protection, because special precautionary measures have to be taken during the production and preparation of the raw material on account of evaporation from the melt, as well as during post-processing, recycling and disposal.
It is known that the manufacture of glass ceramic products occurs in different stages. After the melting and the hot forming, the material is conventionally cooled to below the transformation temperature of the glass. The initial glass is then converted by controlled crystallization into the glass ceramic article. This ceramization takes place in a two-stage temperature process, in which first, by nucleus formation, at a temperature between 600° C. and 800° C. nuclei are generated, conventionally from ZrO2—TiO2 mixed crystals. During the subsequent temperature increase, the high-quartz mixed crystals form on these nuclei at the crystallization temperature of approximately 800 to 900° C. The transformation into the keatite mixed crystals takes place in a temperature region from about 900° C. to 1200° C. As a rule, glass ceramics with keatite mixed crystals as the principal phases are translucent or opaque with a white color and a slightly higher coefficient of thermal expansion than glass ceramics that have high quartz mixed crystals as the principal phase.
To simplify the manufacture of such glass ceramics with an initial float glass, attempts have been made, as described in U.S. Pat. No. 3,804,608, for example, to perform the ceramization as early as in the float bath, to thereby obtain the glass ceramics directly during the float process. However, a temperature gradient over the thickness of the glass strip is produced by the hot float bath and the cold top surface of the glass, which results in crystals that grow aspherically perpendicular to the surface. As a result of the crystallization during the float process, a number of disruptive mechanical and magnetic anisotropies of characteristics are generated.
GB 1 383 201 further describes the floating of Li2O—Al2O3—SiO2 glass ceramics with TiO2, ZrO2 or P2O5 as nucleation agents, which are crystallized by the temperature control during the floating on the molten unwetted metal (tin). This publication describes the necessary temperature control in the float, namely first a rapid cooling followed by a temperature increase, to first perform nucleus formation and then crystallization. In the float, to achieve the high heating and cooling rates, vertical separations extend from the ceiling of the float bath to just above the glass, to separate different temperature zones like individual compartments. The different zones are heated in the tin bath and above the glass strip. Because this publication relates primarily to the manufacture of the floated glass ceramic and not to the unceramized glass that is poured onto the float bath as such, this prior art publication contains no reference to any undesirable surface defects in the glass during the floating. Because the teaching of this prior art patent is limited to the crystallization of the glass that is still in the float, the problem of the formation of undesirable crystals in the glass is not discussed. During the manufacture of float glass from the Li2O—Al2O3—SiO2 system, however, disruptive surface defects occur in the glass and have an adverse effect on the surface quality, as will be described in greater detail below. On account of the economic advantages that can be achieved, moreover, it is also necessary with a glass composition in the Li2O—Al2O3—SiO2 system to manufacture thermally prestressed glass and to realize applications based on it. However, this type of manufacture is not possible with the glass ceramic product described in the above referenced GB patent.